Reclined on a hammock in the Galapagos Islands, University of Utah biology doctoral student Sarah Knutie browsed through the pictures she had taken that morning, stopping occasionally to sip a smoothie. She became distracted when she noticed some nearby Darwin’s finches were up to something. Fascinated, she watched as the birds flew to the laundry line, picking frayed fibers free from the rope, and carried them away to use as liner for their nests.

Her curiosity was piqued. A few days later, she clothespinned a handful of cottonballs to the line. Within days, they were gone. An idea began to form.

That was four years ago. At the time, Knutie was in the Galapagos studying the parasitic nest fly which feasts on the blood of nestlings and mother finches, interrupting the breeding of several types of birds including some endangered species.

The parasite is believed to have invaded Ecuador's Galapagos Islands via boats from the mainland at an unknown time and showed up in large numbers in the 1990s, says University of Utah professor Dale Clayton. Decades later, the flies now infest all land birds there, including most of the 14 species of Darwin's finches, two of which are endangered, Knutie says. Fewer than 100 mangrove finches remain on Isabela Island, and only about 1,620 medium tree finches exist, all on Floreana Island.

According to a study published this week in Current Biology, Knutie, with the help of Clayton and biologists from University of Utah, may have found a solution that will stave off these blood-thirsty parasites: cotton treated with permethrin.

To test their theory, they conducted an experiment at a site named El Garrapatero on the Galapagos' Santa Cruz Island. They built wire-mesh dispensers holding both processed cotton balls treated with one percent permethrin solution and unprocessed cotton balls treated with water as a control.

The cotton balls were collected by at least four species of Darwin's finches: the medium ground finch, small ground finch, small tree finch, and the vegetarian finch. Of the 26 active nests found by biologists, 85 percent contained cotton. Thirteen of those nests had permethrin-treated cotton, nine had untreated cotton, and four had no cotton.

The effect of the cotton on the parasites depended on dosage. Of the eight nests that contained at least one gram of the treated cotton, seven had no parasites and the eighth only had four.

“The problem with the parasite is top priority for the Galapagos National Park and Charles Darwin Research Station, Knutie says. Finding a solution to combat this parasite is really pressing.” While the method has so far only been tested with Galapagos’ birds, she says it might help other species around the world.